1. Field of the Invention
This invention relates to signal-handling apparatus used, for example, in developing and transmitting measurement and/or control signals in instrumentation systems for industrial processes. More particularly, this invention relates to apparatus for translating, storing (as by means of a "memory" device), and reading-out signals the magnitudes of which represent information of a numerical or quantitative nature, e.g. the magnitudes or other measurable status of physical conditions such as pressure, temperature, valve position, and the like.
In many instrumentation systems, it is necessary to develop and to process (i.e. to alter in successive steps or stages) signals representing quantitative information such as the magnitudes, intensities, etc., of specific physical conditions. The required processing or handling of such signals typically involves various kinds of signal translations or transformations, wherein the character of the signal is altered to provide certain desired attributes or to permit certain computations or manipulations to be performed. In many cases, it also is necessary to incorporate as an integral element of the signal-handling function the facility to "remember" the magnitude of a signal level for quite long periods of time, e.g. while associated portions of a control system are devoted to different functions on a time shared basis, or, as a more extreme example, when there has been a breakdown in some part of the equipment, an electrical power failure, or other catastrophic event.
The required memory function in such systems generally will have stringent operational and performance specifications, most particularly with respect to the ability to hold a signal level for long periods of time without significant change for any cause, including drift effects which are associated with certain kinds of known memory elements. In addition, signal-handling apparatus of the type referred to herein must have the capability of providing simple, accurate and non-destructive read-out of the stored signal quantity, whereby the signal information (magnitude) is available in a format which is generally useful in (1) any further processing or signal handling, (2) the presentation of the signal information visually, or (3) the transmission of the signal information to remotely-located associated equipment either on a continuous basis or on an "as called for" basis.
2. Description of the Prior Art
As one example of a prior-art signal-processing apparatus having a memory function of the general class herein considered, reference may be made to U.S. Pat. No. 3,550,014 (D. A. Richardson et al). In that patent, there is disclosed an electronic process controller of the analog type wherein a condition measurement signal and a set-point signal are compared to produce a deviation signal which is processed by rate and reset means to develop a D.C. output signal directed to a flow valve or the like, to position the valve in correspondence with the magnitude of the controller output signal. To accommodate and prepare for switch-over of the controller to manual operation, the controller output signal also is directed to a so-called "memory capacitor" (reference number 74 in the abovementioned patent), to maintain the memory capacitor charged to a level corresponding to the valve control signal.
At switch-over of the controller to manual operation, the memory capacitor referred to above, together with associated signal-producing circuitry, takes over the function of furnishing the valve control signal, maintaining the control signal level at the magnitude it had just before switch-over to manual mode. In practice, the memory capacitor and its associated circuitry are selected to be of high-quality components, capable of holding the capacitor charge with only moderate change over a relatively long period of time. Thus the capacitor can for some time supply a nearly constant, continuous valve signal, substantially equal to the controller valve signal at the moment of switch-over to manual operation.
However, the charge on any such memory capacitor ultimately will dissipate through leakage, even though slowly, so that the valve signal will not remain exactly fixed in magnitude. The result is that the manual signal level must be readjusted from time-to-time, if the controller remains in manual mode for an extended period. Accordingly, there has been a need for a truly satisfactory drift-free memory arrangement to hold the valve signal steady without introducing still other complications or disadvantages. Any alternative memory arrangement particularly should be simple in construction, inexpensive to manufacture, and reliable in performance. Preferably, it should be able to retain the valve signal information even in the face of power failure or other equipment malfunction.
As another example of a prior-art signal-handling apparatus having a memory device, reference may be made to U.S. Pat. No. 3,034,718 (M. P. Freitas et al). That patent describes a process control system of the type which includes a centrally-located digital computer for determining the set points of a number of separate analog controllers. This computer produces set-point adjustment signals which are transmitted periodically, in sequence, to set-point stations associated with respective analog controllers. These set-point stations provide set-point signals for the associated controllers.
Each set-point station also serves the function of a memory device, to store the previously-established set point signal level for use by its associated controller during the times when no signal is received from the computer. The adjustment signals from the computer alter the stored signal level whenever required, and the set point is held at the new adjusted level until the next adjustment signal is received. The set-point signal level also can be adjusted manually, if desired.
The set-point station disclosed in the abovementioned Feitas et al patent basically comprises a mechanical arrangement wherein a constant-speed, reversible motor is responsive to a pulse-length computer signal, and serves to rotationally position a shaft carrying an output potentiometer. When the motor is not energized by the computer signal, it holds the shaft position fixed, thereby "storing" the set-point signal which is read-out by the potentiometer to the associated analog controller. Thus, the reversible motor serves the dual function of rotating the output potentiometer, thereby altering the set-point signal, and also "remembering" the set-point signal by holding the shaft position fixed between adjustment signals from the computer.